Process and apparatus for producing alloyed pn-junctions



June 14, 1966 G. HEISE PROCESS AND APPARATUS FOR PRODUCING ALLOYED PN-JUNCTIONS 4 Sheets-Sheet 1 Filed March 3. 1961 mvzuron Giinther Heise ATTORNEY June 14, 1966 G. HEISE 3,256,120

PROCESS AND APPARATUS FOR PRODUCING ALLOYED PN-JUNCTIONS Filed March 3. 1961 3 I7 I a. A

4 Sheets-Sheet 2 VENTOR G ii at Heise ATTORNE Y G. HEISE June 14, 1966 PROCESS AND APPARATUS FOR PRODUCING ALLOYED PN-JUNCTIONS 4 Sheets-Sheet 3 Filed March 3, 1961 5 Fig. 4

INVENTOR Giinther Heise BY (In ATTORNEY June 14, 1966 a. HEISE 3,256,120

PROCESS AND APPARATUS FOR PRODUCING ALLOYED PN-JUNGTIONS Filed March 3. 1961 4 Sheets-Sheet 4 /2III V Fig.5

INVENTOR Giinther Heise BY H ATTOR N E Y United States Patent 21 Claims. (a. 148-179) The invention relates to a process and apparatus for producing alloyed, preferably large-area, pn-junctions in semiconductor devices, especially diodes or transistors, wherein the alloying material is applied in liquid state onto the surface of the semiconductor.

In the alloying processes now in use several different approaches exist, but none of the known alloying processes is suitable for producing large-area alloyed-junction electrodes, such as are required especially for power tran sistors.

The known alloying process, wherein the alloying material is pressed in solid form onto the semiconductor crystal and then alloyed into the surface of the semiconductor is better adapted for producing small-area junctions. Moreover, in this process the alloying material is frequently found to contain films of oxide and other impurities which, When the alloying material is applied, are directly pressed onto the surface of the semiconductor and cannot subsequently be removed. These impurities then result in defective spots in the junction zone of the alloying front. suitable for producing small-area alloyed junctions with a maximum diameter of 1.5 mm.

Another alloying process teaches the applying of the alloying substance onto the semiconductor body by means of alloy die casting. In this process the alloying die, provided with a bore, is placed upon the semiconductor crystal and then the alloying substance is applied in the form of a small sphere or pellet through the generally cylindrical bore. Finally the alloying is accomplished by heating the alloying die to the alloying temperature. The drawback of the process just described with regard to the manufacture of large-area pn-junctions lies in the fact that, in this alloying process, large-area electrodes require relatively large amounts of alloying material which not infrequently when heated for alloying pass entirely through the semiconductor crystal. Nor can the danger of too deep a penetration of the alloying be avoided simplyby-making the crystal correspondingly thicker, since such an increase in the thickness would impair the electrical properties of the semiconductor device.

In another alloying process, the alloying pellet, which is put onto the cold semiconductor body usually in disc form, is held-down by a kind of pressure plate for the purpose of preventing the alloying material, when fused, from contracting to the form of a sphere under the effect of the surface tension, and consequently from failing to wet the entire zone area of the semiconductor, which should be wetted simultaneously uniformly. However, in this process there is the danger that, in spite of the greatest care, the applied alloying material still may contain traces of impurities, oxides, and other substances which do not permit complete alloying of large-area zones and cause unwetted spots to remain. Moreover, it-is not possible in this process to heat the semiconductor material before alloying to purify the surface of the semiconductor crystal by driving off volatile impurities which likewise might produce unwetted spots.

Putting on the alloying material by means of evaporation does not involve too great difiiculties from a technological point of view, it is true, but the unit production cost thereof is too great. For example, the application by '30 These alloying processes are, at most,

evaporation must be done in a nearly perfect vacuum, so that, apart from the apparatus required, considerable starting delays are necessary. Moreover, after the application by evaporation, a certain time must elapse until the material and its support, unless this latter is watercooled, have cooled to the point where the material put 7 on by evaporation can be taken out without oxidizing.

Finally, a process has become known wherein the alloying material is put onto the surface of the semiconductor in the form of a liquid drop. The use of a capillary tube in this process has the advantage that films of oxide or other films of impurities present in the alloying material are, in most cases, stripped off when the drop leaves the capillary tube. Due to the spherical shape of the drop, however, the alloying process in this method cannot be prevented from proceeding non-uniformly, occurring more deeply in the center of the surface to be alloyed. This leads to an arcuate-shaped path of the alloying front, which is markedly disadvantageous, especially in the case of large-area alloyed electrodes. A

I further drawback in this process consists in the fact that large-area electrodes can be produced only if a correspondingly large amount of alloying material is present, so that here, too, there is the danger of through-penetration. A further disadvantage in this process is the fact that alloyed areas of a definite size are not exactly reproducible since no satisfactory control of the amount of alloying material is guaranteed.

The present invention is directed toward providing a process which eliminates the defects pointed out in the known alloying processes. For this purpose it is proposed according to the invention to bring the alloying material into one or more recesses of an alloy carrier device having recesses which are dimensioned in such a way that their volumes are equal to the volume of the alloyingmaterial to be used and their areas are equal to those of the desired alloy-junction areas. In this process both the semiconductor body and the alloying material are brought to a pre-determined alloying temperature separately from one another, and the alloying material which is at this time contained in the molten state in the recesses of the alloy carrier device is then brought into contact against the semiconductor body and alloyed into it.

The contact of the alloying material with the semiconductor body may be brought about by bringing the alloy carrier device up from below against the semiconductor crystal; but conversely, the semiconductor body may be brought down from above against the alloy carrier device.

A special advantage of the process according to the invention lies in the fact that the entire alloy area is simultaneously wetted by the alloying material immediately upon contact of the alloying substance with the semiconduct-or material. This prevents any arching of the junction due to different wetting times, that is to say, even extensive alloying fronts become plane and extended parallel to the surface of the semiconductor if the semiconductor crystal is precisely oriented. Of special importance, further, is the fact that the amount of alloying material required is no longer determined by the area of the alloying surface to be wetted, but by the volume of the recess in the alloy carrier device. By choosing recesses of small depth which are filled by small amounts of alloying material, economies are effected, in contrast to the known processes, and large-area pn-junctions running parallel to the surface of the semiconductor and having a small depth of alloying are obtained. Moreover, the 'pn-junctions obtained by the process according to the invention are absolutely free of voids and spots of defective alloying, since impurities present in the alloying material and on the surface of the semiconductor evaporate 7 off during heating prior to the actual alloying process,

Patented June 14, 1966 which heating of the alloying material and the semiconductor material is done separately.

Additional objects and advantages of the present invention will become apparent upon consideration of the following description when taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a cross sectional view through apparatus for carrying out the present alloying process.

FIGURE 2 is a cross sectional view through a modified apparatus for alloying several junctions on the same semiconductor.

FIGURES 3 and 4 are cross sectional elevation views through a furnace for simultaneously alloying junctions on separate semiconductor devices.

FIGURES 5 and 6 are, respectively, a cross-sectional view and an elevation view of a modified furnace and material uniting means for carrying out the present alloying process.

FIGURE 1 shows the basic arrangement of a vertical alloying furnace for carrying out the process according to the invention. The semiconductor body 1 to be alloyed is introduced from the top into a crystal receiver 2 until it comes to rest on a shoulder 3 in the inside of the crystal receiver. Subsequently, the semiconductor crystal is weighted by an additional weight 4.

The alloying material 5, required for the alloying process, is placed in a recess 6 of an alloy carrier 7, and the alloy carrier 7 is raised from below into the crystal receiver 2. As may be seen from FIG. 1, the alloy carrier 7 is mounted on a ceramic pipe 8 at whose upper end a thermocouple 9 is arranged for continuous temperature measurement and/ or control.

At first the alloy carrier 7 is partly raised into the crystal receiver 2 only to a point below the bottom surface of the semiconductor crystal 1. This first position taken by the alloy carrier 7 will be referred to as the heating position. In this heating position, both the semiconductor body 1 and the alloying material 5 are brought to the predetermined alloying temperature separately from one another. This heating carried out prior to alloying is to effect evaporation of any impurities present in the alloying material and on the surface of the semiconductor. The heating time is about 2 minutes; if an indium alloying material is used, a heating temperature of 400500 C., for example, is advisable. The impurities liberated during heating can escape from the crystal receiver 2 to the outside 'by way of slots 10 provided in the crystal receiver for the removal of the evaporation products.

The temperatures required both for evaporation and for alloying are produced by a heating coil 11, which is fixed on a heating coil carrier 12.

After the heating is finished, the alloy carrier 7 is raised to the point where its upper surface touches the semiconductor crystal 1. In order to determine when this contact takes place, it is advisable to bring the alloy carrier 7 up into the crystal receiver 2 to the point where the semiconductor body 1 is slightly raised. At this time the molten alloying substance wets the entire alloying surface of the semiconductor crystal at the same time, and due to the uniform thickness of the layer, the alloying material is alloyed very uniformly with the semiconductor material. The weight 4 must be such as to overcome the surface tension of the liquid alloying substance so that the semiconductor body 1 will not be lifted from the front surface of the alloy carrier due to the surface tension prevailing in the liquid alloying material.

After the alloying is finished, the alloyed material is cooled and the alloy carrier 7 is lowered out of the crystal receiver 2 when the temperature has dropped below the melting point of the alloying material. For indium, this temperature lies at about 156 C. Upon retracting the alloy carrier from the crystal receiver, the alloying process is finished.

The alloyed areas obtained by the process according to the invention are exactly reproducible, since the surface of the semiconductor wetted 'by the alloy material always corresponds exactly to the area and contour predetermined by the recess 6. The same is true for the depth of alloying, since this is predetermined :by the depth of the recess 6.

FIGURE 2 shows the embodiment of an alloying furnace which differs from that of FIG. 1 in the design of the alloy carrier. In contrast to the alloy carrier 7 of FIG. 1, the alloy carrier 7 in this embodiment is designed in several parts, making possible the production of not only one but of several alloyed junctions of any desired shape.

In the alloy carrier 7 of FIG. 2 three carrier parts are included, namely a central carrier part 13 around which two further carrier parts 14 and 15 are concentrically arranged. The carrier parts 13 and 14 are backed by springs 16 and 17 which are dimensioned in such a way that, when the alloy carrier is moved upward, the contact of the individual carrier parts with the bottom surface of the semiconductor body takes place not simultaneously but successively, the central carrier part 13 with its upper surface touching the semiconductor crystal first. Due to this contact and during the upward movement of the alloy carrier, the middle spring 16 is compressed, thereby permitting the contact of the second carrier part 14 with the semiconductor body. The upward movement of the alloy carrier is finished when the upper surfaces of all carrier parts lie in one plane and all carrier parts touch the semiconductor body. It is advisable here, too, to check by slightly raising the semiconductor body upwardly to be sure that the desired contact between the semiconductor material and the alloying material has taken place.

The alloy carrier 7 shown in FIG. 2 may, of course, also consist of more than three individual carriers, the number of the carrier parts to be employed depending in general upon the number of electrodes to be alloyed. The division of the alloy carrier into individual carriers is useful where several electrodes are to be alloyed onto the same semiconductor surface next to one another.

Just as in the case of the alloy carrier 7 of FIG. 1, the individual carrier parts of the alloy carrier 7 are provided with recesses whose shapes and sizes depend upon the dimensions of the alloyed electrodes to be produced. In the embodiment of FIG. 2, the recesses of the carrier parts are such as to result in annular electrodes concentrically arranged around the central alloyed electrode. In this embodiment, the emitter electrode is arranged around the base electrode in the center of the semiconductor body, which emitter electrode is, in turn, concentrically surrounded by the outer base electrode.

The alloying device shown in FIGS. 3 and 4 offers the possibility of alloying several semiconductor units at the same time, using a sliding furnace. In this process, a quartz pipe 19, surrounded by a heater and having protective gas flowing through, is placed over the material to be alloyed. In the embodiment of FIGS. 3 and 4, two crystal receivers 2 are arranged next to one another in the alloying die 18, although the number of alloying couples appearing in the longitudinal direction illustrated in FIG. 4 depends upon the size of the heating zone of the alloying furnace, that is to say, upon the length of the piece of quartz pipe wherein the temperature desired at any given time can be uniformly maintained.

The contact of the semiconductor body 1 with the alloying material 5 does not come about as in the devices of FIGS. 1 and 2, by an upward movement of the alloy carrier 7" but, conversely, by a lowering of the crystal receivers containing the semiconductor crystals and the holding weights 4.

In this process, special attention has to be given to the fact that, due to the required separate heating step, the contact of semiconductor material and alloying material may take place only after such heating. Thus, a mechanism must be provided which assures that the alloying material and the semiconductor material are kept separate during the heating, but brought into contact with one another after the heating. For this purpose, a slide 20 is provided which during the heating is pushed between the flange 21 of the crystal receivers 2 and the surface 22 of the alloying die 18.

After the heating is finished, the slide 20 is pulled out to the right (FIG. 3), thereby successively freeing two crystal receivers at one time. The freed crystal receivers glide slowly downward along the oblique plane 23 until the crystals come to rest-on the alloy carrier 7". Thus the alloying material is brought in cont-act with the semiconductor body and the alloying process initiated.

FIGURES 5 and 6 show an embodiment wherein the alloying is done in a continuous-heating tunnel-furnace. In this device, too, arrangements are made which take care that, after the'separate heating time is finished, the crystal receivers, including weights and crystal bodies, are lowered downwardly,

The entire arrangement of FIG. 5 must be thought of as moving from the rear to the front on a belt 24. In the tunnel of the continuous-heating furnace, not shown here, and at a very definite spot, namely that at which the alloying process is to be initiated, a stationary arm 25 is disposed to exert a torque upon rotatable arms 26. Thus, when the device shown'in FIG. 5, in its passage through the tunnel, reaches the spot where the stationary arm 25 is located, the rotatable arm 26 on the crystal receiver is turned, due to the forward movement of the device on the belt, far enough to the rear that it can glide without hindrance past the stationary arm 25. The crystal receiver 2" thus rotated in the die receiver 27 causes the crystal receiver to descend according to FIG. 6, lowering the arms 26 and 28 into the opposed notches 29 of the die receiver 27, thereby bringing together the crystal 1 with the alloying material 5, the latter being supported on the alloy carrier 7"". The further alloying process proceeds as in the examples already described.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

I claim:

1. The process for producing pn-junctions of an alloying material with a semiconductor, including the steps of heating the ssemiconductor and the alloying material, while the same are mutually separated from each other,

' to an alloying temperatureat which the alloying material is molten; confining the molten alloying material to the shape of the desired junction in consequence of which the volume of the confined molten liquid is equal to the volume of the alloying material desired for the alloyed electrode of the finished junction and the surface area of the confined molten liquid is equal to the area desired for the alloyed electrode of the finished junction; bringing the molten alloying material and the semiconductor surface together while the former is still confined as aforesaid; and cooling the semiconductor and the alloying material.

2. The process set forth in claim 1, wherein the alloyingmaterial is confined within a recess; said recess being located in the upper surface of a support and having an area equal to the area desired for the alloyed electrode; the depth of said recess being such as to give in connection with the area of the recess the desired volume of the alloying material.

3. The process set forth in claim 1, wherein the semiconductor is supported stationary and the molten and confined alloying material is heated below its surface and then raised into contact therewith.

'4. The process set forth in claim 1, wherein the alloying material is heated and melted in a receptacle and the semiconductor is then lowered from above into contact with the molten material. 7

5. The process as set forth in claim 1, wherein both the semiconductor and the alloying material are main-- tainedat said alloying temperature for about 2 minutes before being brought together.

6. Apparatus for producing pn-junction devices by alloying on the surface of a semiconductor an alloying material, comprising, in combination; a heat producing furnace; first support means in said furnace for supporting said semiconductor with its surface disposed horizontally; second support means in said furnace having an upper surface locatedbelow said first support means, and said second support means having at least one alloyingmaterial receiving recess in its upper surface; and mechanical means for approaching one of said support means toward the other along a vertical axis to bring said alloying material into contact with the surface of said semiconductor.

7. Apparatus as set forth in claim 6, wherein said furnace comprises a member having a vertical bore therethrou-gh including an upper bore separated from a lower bore by a shoulder comprising said second support means being reciprocably supported in the lower bore; and weight means in said upper bore adapted to yieldably urge said semiconductor'downwardly.

8. Apparatus as set forth in claim 7, wherein said member has openings communicating through its side walls from said bore through which vaporized impurities can escape.

9. Apparatus as set forth in claim 7, wherein said fur nace further includes heating means disposed around th side walls of said member.

10. Apparatus as set forth in claim 6, wherein the shape of said recess as parallel with said upper surface is the shape of the desired junction on the semiconductor and the depth of the recess is equal to the thickness of the alloying material to be united with the semiconductor.

11. Apparatus as set forth in claim 6, wherein said second support means includes a ceramic mount anda thermocouple for measuring the temperature in the vicinity of the recess.

12. Apparatus as set forth in claim 6, wherein said second support means comprises at least two parts each having at its upper end a surface having a recess therein for applying alloying material to produce plural alloyed junctions on dilfercent zones of said semiconductor.

13. Apparatus as set forth in claim 12, wherein each of said parts is annular and disposed concentrically with the other parts about said vertical axis, and each of said recesses in the parts is concentric with said axis.

14. Apparatus as set forth in claim 13, one of said parts being fixed on said second support means and the other parts each being resiliently mounted thereon by a spring means.

15. Apparatus as set forth in claim 14, each spring means supporting the upper surface of the associated part at an elevation different from that of any other upper surface of a part and above the upper surface of said fixed part, whereby as said support means are moved toward each other, said upper surfaces will contact the semiconductor surface in successive sequence.

16. Apparatus as set forth'in claim 15, wherein, the outermost part is fixed, and the center part contacts the semiconductor surface first.

17. Apparatus as set forth in claim 6, wherein said furnace comprises a member having a vertical bore therein, said second support means being located in said bore, and said first support means being reciprocably carried in the bore thereabove; means supporting said semiconductor near the lower end of said first support means; and said mechanical means comprising a wedge shaped incline on said member engaging said first support means, and means for moving said incline thereon to lower the first support means and move the semiconductor into contact with the alloying material.

-18. Apparatus as set forth in claim 17, said incline com-prising diagonal slots in said member, the bore and the first supporting means being of circular cross-section, and the latter having radially extending arms riding in said slots and changing the elevation of the first supporting means when the latter is rotated in the bore.

19. Apparatus as set forth in claim 17, wherein said member has openings communicating through its side walls from said bore through which vaporized impurities can escape.

20. The process as set forth in claim 1, wherein the semiconductor, for purposes of carrying out the alloying, is placed into a stepped recess of an alloying apparatus, and wherein the semiconductor, during alloying, is weighted when the alloying material is brought toward the semiconductor from below.

21. A process for making pn-junctions of an alloying material with a semiconductor, comprising the steps of: heating the semiconductor and the alloying material to an alloying temperature for liquefying the alloying material and also for evaporating impurities which are present in the alloying material and on the suuface of the semiconductor, the semiconductor and the alloying material, during said heating thereof, being physically spaced apart from each other and the space between the alloying material and the semiconductor being an open space in consequence of which the evaporation products which result from the heating may escape; confining the liquefied alloying material to the shape of the desired junction in consequence of which the volume of the confined molten liquid is equal to the volume of the alloying material desired for the alloyed electrode of the finished junction and the surface area of the confined molten liquid is equal to the area desired for the alloyed electrode of the finished junction; bringing the liquefied alloying material and the semiconductor surface together while the former is still confined as aforesaid; and cooling the semiconductor and the alloying material.

References Cited by the Examiner UNITED STATES PATENTS 2,746,742 5/ 1956 Comley 2665 2,752,148 6/1956 Kincaid et al 2665 2,857,296 10/1958 Farris 148-1.5 2,881,103 4/1959 Brand et al. 1481.5 2,893,901 7/1959 Lehovec 1481.5 2,900,287 8/1959 Bestler et a1 148-15 3,043,722 7/ 1962 Houben et a1. 148179 3,097,976 7/1963 Lehovec 148179 DAVID L. RECK, Primary Examiner.

RAY K. WINDHAM, Examiner.

C. N. LOVELL, M. A. CIOMEK, R. O. DEAN,

Assistant Examiners. 

1. THE PROCESS FOR PRODUCING PN-JUNCTIONS OF AN ALLOYING MATERIAL WITH A SEMICONDUCTOR, INCLUDING THE STEPS OF HEATING THE SSEMICONDUCTOR AND THE ALLOYING MATERIAL, WHILE THE SAME ARE MUTUALLY SEPARATED FRM EACH OTHER, TO AN ALLOYING TEMPERATURE AT WHICH THE ALLOYING MATERIAL IS MOLTEN; DONFINING THE MOLTEN ALLOYING MATERIAL TO THE SHAPE OF THE DESIRED JUNCTION IN CONSEQUENCE OF WHICH THE VOLUME OF THE CONFINED MOLTEN LIQUID IS EQUAL TO THE VOLUME OF THE ALLOYING MATERIAL DESIRED FOR THE ALLOYED ELECTRODE OF THE FININSHED JUNCTION AND THE SURFACE AREA OF THE CONFINED MOLTEN LIQUID IS EQUAL TO THE AREA DESIRED FOR THE ALLOYED ELECTRODE OF THE FININSHED JUNCTION; BRINGING THE MOLTEN ALLOYING MATERIAL AND THE SEMICONDUCTOR SURFACE TOGETHER WHILE THE FORMER IS STIALL CONFINED AS AFORESAID; AND COOLING THE SEMICONDUCTOR AND THE ALLOYING MATERIAL. 